New techniques improve characterization of DWDM passive components
An erbium-doped fiber tunable laser source offers several advantages over common tunable lasers for testing a variety of components.
MICHAEL CARLSON, EXFO E.O. Engineering Inc.
Data-transmission systems are relying more and more on wavelength-division multiplexing (WDM) technology. In fact, WDM has become dense-WDM (generally meaning 200-GHz interchannel spacing) and we are starting to hear the term ultra-dense-WDM (50-GHz interchannel spacing). Component and system manufacturers are starting to master the techniques required to pack an astounding number of wavelengths into the transmission band.
The passive components used in these networks have very narrow bandwidths and require wavelength-resolution characterization throughout all phases of product development, whether it is R&D, qualification, or manufacturing. Testing these components is becoming a serious bottleneck, mostly because the test equipment does not have the required wavelength resolution, or it is too costly to purchase the testing solutions currently on the market, or the wavelength accuracy is insufficient, or it is difficult to test multiple channel devices. For new component manufacturers just breaking into the DWDM business, the difficulties are even greater. Manufacturers are looking for ways to cut the job of testing DWDM passive components down to size.
There`s a new type of tunable laser source that`s ideal for passive-component testing and can be used in an innovative way to test DWDMs.
Why a new laser is necessary
Most manufacturers now use some variation of the external cavity (EC) tunable laser source for high wavelength-resolution characterization of their passive components. Not only are these sources very costly, but they have inherent characteristics that make them less than ideal for testing passive components. Characteristics such as source spontaneous emissions (SSE), side-mode suppression (SMS), coherence length, and wavelength accuracy all have to be either controlled or corrected when testing passive components.
Surprisingly enough, there is a lower-cost alternative, the erbium-doped fiber tunable laser source (edf-tls), which is much better suited for the job. The edf-tls is based on a fiber ring cavity (see Fig. 1). Power from the 980-nm pump laser is coupled into the cavity containing, among other components, erbium-doped fiber and a tunable interference filter. A motorized positioning assembly, controlling the angle of the filter, provides continuous-wavelength tuning across the erbium-doped fiber-amplifier band. Although not a new technology, recent improvements in fiber-optic components and a better understanding of fiber lasers have allowed instrument manufacturers to produce edf-tlss with impressive performance specifications.
Performance comparisons
The output from the edf-tls is shown in Figure 2. For easy comparison, the output spectrum of a typical EC-TLS is also shown. The first thing you will notice is the difference in noise level (or spontaneous emissions) of the two sources. You will also notice the complete lack of sidemodes on the edf-tls spectrum. For anyone who is using a tunable laser source for testing passive components, the advantage of the edf-tls will be obvious. The more than 60-dB noise suppression (in this case, 66 dB) of the edf-tls means that you can measure with almost 60-dB dynamic range using a simple power meter. With an EC-TLS, this would be impossible unless a synchronized and calibrated tracking filter was cascaded at the output of the TLS, adding complexity and cost and reducing the reliability of the measurement system.
Figure 3 shows the loss measurement results of a test on a multiplexer channel using two different tunable laser sources (EC and EDF) and a high-sensitivity power meter. In addition to a reduction in dynamic range, the measurements with the EC-TLS also give erroneous bandwidth and filter slope measurements.
The spectral linewidth of a tunable laser source is another important parameter when testing passive components. A narrow linewidth will allow high-resolution spectral characterization. But if the linewidth is too narrow, multipath reflections in the measurement setup will result in unstable and nonrepeatable power measurements. The instability is due to multipath interference, and this effect increases as the linewidth decreases.
A commonly used technique to increase the effective linewidth of an EC-TLS is coherence control. There are different methods for doing coherence control, but essentially the laser wavelength is dithered around a central value. The dither modulation frequency is normally in the order of several kilohertz. This works very well for slow power measurements, but if fast synchronized measurements are required, the coherence control modulation could introduce other undesired effects.
Another approach is to use a source whose inherent effective line-width is quite large. A commercialized edf-tls system has been designed to exhibit an effective spectral width of approximately 1 GHz (8 pm at 1550 nm) and is very close to the ideal spectral distribution for DWDM passive-component testing. In this sense, and especially when compared to an EC-TLS, it can be considered a medium-coherence source.
Continuous tuning
Normally, when characterizing passive DWDM components, measurements should be made as quickly as possible across a specified wavelength range. To do this, a continuously tunable laser is needed. It is very difficult to produce an EC-TLS that does not exhibit some sort of mode hopping or signal dropout when tuning from one wavelength to the next or while in a continuous sweep. Significant gains have been realized recently, but the complexity and therefore the cost to the consumer reflect these gains in performance.
If properly designed and with particular attention paid to polarization effects in the fiber-ring cavity, it is relatively simple to design the edf-tls to be continuously tunable with no signal dropouts. This design reduces the complexity, increases the reliability, and keeps the cost down.
Wavelength accuracy
Based on recent market research, customers are looking for absolute wavelength accuracy of 0.01 nm or better for characterizing DWDM components. As a result, instrument manufacturers have put tremendous effort into improving the accuracy and repeatability of their products. Again, progress has been made, but it must be pointed out that with the current approach of using precision mechanical positioning of a grating or other component, a tunable laser source can never be used as an absolute wavelength reference. It would be too risky to rely on the source wavelength calibration for narrow-channel DWDM components. Unless checked regularly, there is no way of knowing when it may go out of calibration. An alternative and independent wavelength reference method is required.
The most common wavelength referencing technique used today is the Michelson interferometer-based wavelength meters. They are available as dedicated benchtop units or as modular instruments and will provide the assurance to keep your tunable laser source dependable, whichever type you decide to use. However, it`s easy to question the wisdom of paying for the most accurate TLS when it is probably not well suited for the test being performed and a wavelength reference measurement capability will be needed anyway.
Putting it all together
When testing DWDM components with an edf-tls, the objective is to scan the source while taking measurements with a power meter and accurately plotting these measurements versus wavelength. Once this measurement is accomplished, power meters can be added to measure multiple channels at the same time. If a suitable instrument system is selected, as many power meters as optical ports could be added and a multi-port device could be completely characterized with a single scan of the TLS. The instruments required are shown in Figure 4. It is important to note that in a modular test system, it is simple and cost-effective to add multichannel power meters to expand the system to cover additional channels or to even add polarization-dependent loss (PDL) and optical return loss (ORL) to the measurement system.
In Figure 4`s system, the tunable laser will scan from 1520 to 1570 nm while the fast acquisition power meters measure the power transmitted through each demux channel. The wavelength meter module will provide absolute wavelength reference, and loss can be calculated from either a reference measurement or a reference channel. It is worth noting that other absolute wavelength referencing and linearizing techniques are also being developed. These referencing techniques can guarantee wavelength accuracy to 5 nm or better.
To perform this measurement sequence in an efficient manner, automatic software control is required. Ideally the software should provide the flexibility to:
select a 16- or 32-channel multiplexer part number and automatically configure the test parameters.
automatically perform pass/fail analysis.
permit repeated testing for drift analysis.
automatically calculate insertion loss, bandwidth at different power levels, center wavelength, crosstalk, PDL, and ORL.
save data to a database and print reports.
provide instructions and guidance to operators.
analyze different filter types such as notch filters, bandpass, shortpass, and long pass.
Advantages to consider
The edf-tls offers a number of important advantages over the traditional EC-TLS when used for testing passive components, notably very low noise level, lack of side-modes, continuous tuning, and medium coherence. Using this source with multichannel power meters and an absolute wavelength reference permits accurate and rapid characterization of passive DWDM components, including high channel-count multiplexers and demultiplexers. u
Michael Carlson is scientific product manager at EXFO E.O. Engineering Inc. (Vanier, QC, Canada).